The development of inorganic films or membranes which are selectively permeable to specific gases and are able to withstand the adverse environments encountered in most processes is becoming increasingly important. Such membranes must be stable at high temperatures and resistant to chemical attack to be suitable for use in a combined process involving a catalytic reaction and product separation. Through the use of such selective permeation membranes, the yield of catalytic processes which are currently restricted by thermodynamic equilibrium can be significantly improved.
Inorganic membranes typically cost ten to one hundred times more than the commonly used polymer membranes. Because of their high cost the commercialization of inorganic membranes depends critically on their permeance or productivity. For a given processing duty the membrane capital cost is approximately inversely proportional to the membrane permeance which, in turn, depends inversely on the effective thickness of the membrane.
In U.S. Pat. No. 4,902,307, now U.S. Pat. No. 5,453,298 issued Sep. 26, 1995 (hereinafter referred to as the "PATENT"), prior art concerning inorganic membranes is reviewed. In the PATENT a technique, called the "opposing reactants deposition technique" for depositing internal SiO.sub.2 layers within porous Vycor tubes ("sandwich configuration") is described. The deposition reaction set forth in the PATENT was the oxidation of silane (SiH.sub.4). Briefly, that technique required the flow of one reactants, silane, inside the support tube, and the other reactant, oxygen, outside the support tube. The reactants diffuse in opposite directions and meet at some intermediate region within the tube wall where the film deposition reaction takes place. Once all the open pore paths are blocked by the deposited SiO.sub.2, a barrier is formed which is highly selective to hydrogen permeation.
The PATENT disclosed that the thickness of the deposited film varied inversely with the reaction rate. Thus, increasing the temperature and the concentration of the reactants increased the reaction rate and resulted in thinner films. The practical temperature for SiO.sub.2 deposition by oxidation of silane was found to be in the range of from 400.degree. to 500.degree. C. Below this range the reaction rate was reported as too slow, and the deposition film too thick; above this range silane would decompose thermally throughout the porous substrate forming a thick layer of silicon of extremely low permeability to all gases, including hydrogen.
In the PATENT, the SiO.sub.2 films deposited at about 450.degree. C. had hydrogen permeation rate coefficients of about 0.2 cm.sup.3 /cm.sup.2 -min-atm, and H.sub.2 :N.sub.2 permeation rate ratios of about 3000, both measured at 450.degree. C. immediately after deposition. Subsequent exposure to high temperatures, especially in the presence of water vapor, caused the permeability to hydrogen to decrease considerably. For example, it was reported that heating at 600.degree. C. for one day in the presence of water vapor decreased the permeation rate by a factor of 3, and heating at 700.degree. C. for an additional day led to a further 30% reduction. The decrease in permeability was attributed to densification of the SiO.sub.2 film.
Ser. No. 221,873, filed Apr. 1, 1994, now U.S. Pat. No. 5,453,298 issued Sep. 26, 1995 the formation (hereinafter) referred to as the "APPLICATION") is concerned with of films of SiO.sub.2, B.sub.2 O.sub.3, TiO.sub.2, Al.sub.2 O.sub.3 and mixtures thereof. These films were first formed as layers or films within the walls of a porous substrate tube by the hydrolysis of the respective halides (chlorides, bromides or iodides) by the "one-sided flow deposition method" or the "alternating deposition method" rather than by the opposing reactants deposition technique described in the PATENT. In the APPLICATION a SiO.sub.2 layer is deposited by the reaction of silicon tetrachloride, SiCl.sub.4, with water vapor, or by the reaction of the compound hexachlorodisiloxane or Cl.sub.3 SiOSiCl.sub.3 with water vapor, or by the reaction of the compound octachlorotrisiloxane or Cl.sub.3 SiOSiCl.sub.2 OSiCl.sub.3 with water vapor. The APPLICATION discloses that the latter two silicon compounds react faster with the Vycor.TM. glass substrate, and form thinner SiO.sub.2 layers which have higher diffusivities than layers formed using silicon tetrachloride, SiCl.sub.4.
The APPLICATION also disclosed that a SiO.sub.2 layer can be deposited by the reaction of chlorosilanes, i.e. SiHxCl.sub.4-x (x=1, 2 or 3) with a mixture of water vapor and oxygen.
For brevity, the PATENT and the APPLICATION are hereinafter referred to collectively as the "REFERENCES". One of the inventors of the REFERENCES is also one of the inventors of this invention. The REFERENCES are hereby incorporated herein by reference.
The reaction between the halide reactants and the water vapor to form the oxide deposit can take place in the gas phase outside of the porous tube wall, and on the pore surface of the tube wall. Unfortunately, reactions in the gas phase produce particles which adhere to the tube wall. Such deposits form a relatively thick layer which decreases the membrane permeance and causes thermomechanical stresses that can result in cracks and membrane failure. This invention, and those of the REFERENCES, are not directed to deposition on the outside or inside surface of the porous tube but rather inside the porous tube wall, i.e. in a thin region somewhere between the outside surface and the inside surface of the porous tube, which is referred to herein as the "pore surface". Therefore, this invention and the APPLICATION seek to maximize oxide deposition on the pore surface and minimize oxide deposition on the external tube surface.
With the opposing reactants deposition technique of the PATENT it was possible to avoid or at least greatly decrease the generation of particles in the gas phase and the formation of an external deposit. However, the pore surface deposits, i.e. internal deposits between the external outside surface and external inside surface of the tube, formed by the opposing reactants deposition technique are relatively thick which is undesirable. The one-sided flow deposition method of the APPLICATION produced thinner and more permeable deposit layers and was a significant improvement over the opposing reactants deposition technique of the PATENT. However, in the one-sided flow deposition method, particle formation in the gas phase was not completely avoided but was greatly minimized by using low concentrations of reactants and carefully controlling the deposition time.
The APPLICATION further disclosed that the reactants concentrations decrease in the flow direction parallel to the axis of the porous tube, from entry point to exit point, due to the chemical reaction. As a result, the deposit layer at the upstream section of the porous tube, i.e. at the point where the reactants first contact the porous tube section, was thicker than at the downstream section, i.e. at the point where the reactants last contact the porous tube section. This caused the resulting membrane permeance to be lower than it would be for a layer of uniform thickness. Accordingly, the APPLICATION improved upon the one-sided deposition technique by the alternating flow deposition method. In the alternating flow deposition method the porous tube was first evacuated both on its inside and outside, the halide reactant was then introduced over the inside or outside surface of the porous tube, and time was allowed for the halide reactant to enter and become grafted on the pore surface.
In the APPLICATION, the porous tube was then purged with an inert carrier gas, e.g. N.sub.2 to remove the halide reactant from the space inside or outside of the porous tube. After purging was completed, water vapor was allowed to flow over the same surface of the porous tube as the halide reactant previously flowed and hydrolysis of the halide on the pore surface occurred. After hydrolysis was completed, the porous tube was again purged with an inert carrier gas. These steps were repeated until the desired permeation selectivity of the deposit layer was achieved.
An important parameter in the alternating flow deposition method was the dosage of halide introduced into the evacuated porous tube per unit area of internal surface, i.e. the surface formed by the inside diameter of the tube. It was disclosed that the dosage should be sufficiently small to limit the depth of penetration in the pores of porous tube segment thereby limiting the thickness of the ultimately formed oxide layer. The smaller the halide dosage in a cycle, the thinner and the more permeable the oxide membrane formed in the cycle. Also, the smaller the halide dosage per cycle, the larger the number of cycles required to obtain the desired permeation selectivity. However, the improvement in the membrane permeation coefficient diminished and became insignificant when the dosage was decreased below a certain level. Control of the dosage was achieved by controlling the concentration of the halide flowed into the porous tube.
Non-limiting examples of preferred porous supports are substrates made of Vycor.TM. Brand Glass No. 7930 and Al.sub.2 O.sub.3. The Vycor.TM. glass used in the REFERENCES and herein as the porous substrate is a porous borosilicate glass with over 96% SiO.sub.2, 3% B.sub.2 O.sub.3, and smaller amounts of Al.sub.2 O.sub.3, and other oxides. The mean pore diameter is in the range of 25.ANG. to 120.ANG. depending on the manufacturing conditions Tubes made from Vycor.TM. Brand Glass No. 7930 used in most of the experiments in the REFERENCES and herein, had mean pore diameter 40.ANG., internal diameter (ID) 4.8-5 mm, and external diameter (OD) 6.8 to 7.2 mm.